1. Field of the Invention
The present invention relates to a transfer apparatus having an image carrier for holding thereon an image formed by a developer charged with a prescribed polarity and transfer means disposed opposite the image carrier, wherein a recording medium is fed to an area where the image carrier and the transfer means oppose to each other, and the developer image on the image carrier is transferred the recording medium by using a transfer electric field formed by the transfer means. The invention is applicable to print sections in digital copiers and facsimile machines as well as to digital printers, plotters, and the like.
2. Description of the Related Art
As an image forming apparatus for outputting an image signal onto a recording medium such as paper in the form of a visible image, Japanese Examined Patent Publication JP-B2 2733609, for example, discloses an image forming apparatus that forms an image by transferring a developer image (toner image) on an image carrier (photoconductor drum) to a recording medium using a transfer roller.
In such an image forming apparatus as exemplified by the above prior art, a toner image developed on a photoconductor drum is transferred to a recording medium, i.e., paper surface, by pressing an elastic transfer roller having a medium to high resistance in contacting relationship to the paper. Such contact transfer techniques have the excellent characteristics of being able to produce images of good quality without generating ozone during the process.
In such image forming apparatus, when transferring the toner image developed on the photoconductor drum, a transfer electric field is formed by applying a transfer voltage to the transfer roller. Especially, according to the prior art, a good transfer is achieved by specifying the amounts of charge to be supplied to toner-image areas and non-toner-image areas when transferring the toner image from the photoconductor drum.
With this arrangement, however, it may become difficult to transfer the toner image securely to the paper, depending on the condition of the toner.
For example, when the amount of charge of the toner used is very small and the amount of toner adhering to the photoconductor drum is also very small, reverse transfer may occur and good transfer may not be obtained, thus resulting in a transfer failure. If this happens, various troubles occur, such as an insufficient density or a dropout in a solidly shaded area, the inability to reproduce the desired halftone because of a transfer failure of a screened halftone image, and in the case of a color image forming apparatus, difficulty in reproducing the correct color.
Further, if the transfer is to be performed without neutralizing the charge the toner has, it is required to form a transfer electric field strong enough to overcome the image force being exerted by the charge the toner has, and in this case, an extremely large transfer electric field becomes necessary.
If such a large electric field is formed, since the amount of charge of the toner held to the photoconductor drum is not constant but varies across the surface of the drum, toner particles with smaller charge amounts are easily attracted by the charge and scattered onto the paper on the upstream side of transfer area, causing image degradation.